Not really, it was just Kabutos opinion. In the official databook released it was stated that the 3rd Hokage was the strongest. The 4th would have probably become the strongest but died too young obviously.

@One_Piece_God: No, you still can't fanwank Hashirama. At this point you could probably say he's better than any of the current kage and better than EMS Madara, but saying he's the best ninja in the whole series except for Rikudo sennin is bull.